The present invention relates to a process for producing L-ascorbic acid. More particularly, the present invention relates to a continuous process for producing L-ascorbic acid that minimizes decomposition of the L-ascorbic acid product formed and allows for unreacted starting material to be recycled back into the reaction mix.
L-Ascorbic acid (vitamin C) is produced commercially by combined chemical and fermentation processes starting from glucose or sorbose. A common intermediate generated in the commercial process is 2-keto-L-gulonic acid (KLG), or its protected form, diacetone-2-keto-L-gulonic acid. The conversion of 2-keto-L-gulonic acid to L-ascorbic acid may be carried out by esterification with methanol, followed by cyclization using stoichiometric amounts of a base, in a methodology derived from the original Reichstein process (T. Reichstein, A. Grussner, Helv. Chim. Acta 17, p. 311-328, 1934). Alternatively, diacetone-2-keto-L-gulonic acid may be cyclized directly, with a loss of acetone followed by consecutive lactonization and enolization, to form ascorbic acid. Direct cyclization of diacetone-2-keto-L-gulonic acid requires extensive purification for recovery of the acetone and other byproducts generated.
Additional modifications to the Reichstein process have focused on removal or simplification of many of the chemical processing steps required for the production of 2-keto-L-gulonic acid. Improvements include controlled esterification of 2-keto-L-gulonic acid and subsequent removal of unesterified starting material (U.S. Pat. No. 5,128,487), as well as improved integration of esterification with subsequent cyclization (U.S. Pat. No. 5,391,770).
Efforts have also been directed to acid catalysis (e.g. U.S. Pat. No. 2,462,251; GB 1,222,322, GB 2,034,315; DE 3843389, WO 99/07691; and WO 00/46216). Acid catalysis employs 2-keto-L-gulonic acid in its acid form, and thus removes the need for the generation of the ester and subsequent steps requiring the addition of stoichiometric base for cyclization with reprotonation of the ascorbate salt to isolate the product in its acid form. As the reprotonation step generates a stoichiometric amount of salt byproduct, acid catalysis can result in significant reduction in waste and processing costs with relatively high yields ( greater than 80%) of L-ascorbic acid product. Modifications to improve the process such as the use of organic solvents and surfactants have been described (see e.g. U.S. Pat. No. 5,744,618; WO 98/00839; and JP-B 73015931). Although an improvement over the original Reichstein process, acid catalysis still requires significant handling, recycling, and purfication steps to obtain a high yield of ascorbic acid.
An alternative means of producing ascorbic acid from 2-keto-L-gulonic acid involves an aqueous intramolecular cyclization process without the use of copious amounts of acid catalysts (T. Reichstein, Helv. Chim. Acta 17, 1934, pp. 311-328 and BP 428,815). Although aqueous cyclization does not require the extensive purification steps associated with acid catalysis, non-acid catalyzed intramolecular cyclization is associated with relatively low yields. For example, 2-keto-L-gulonic acid may be heated in water saturated with carbon dioxide with 50% yield after fractional crystallization (U.S. Pat. No. 2,265,121). Also, 2-keto-L-gulonic acid or derivatives of 2-keto-L-gulonic acid may be heated to 130-140xc2x0 C. in water to generate ascorbic acid with yields approximating 50% (U.S. Pat. No. 2,491,065).
Numerous attempts at direct cyclization processes for keto-L-gulonic acid (KLG) and derivatives thereof have been proposed in which the final product is isolated from the cyclization stream by removal of the solvent. Purification of the L-ascorbic acid product is hampered, however, due to the instability of L-ascorbic acid product in aqueous or acid reaction solutions (e.g. P. P. Regna and B. P. Caldwell, J. Am. Chem. Soc., 66, pp. 246-250, 1944), especially when the reaction is operated such that conversion of the 2-keto-L-gulonic acid starting material is nearly complete. Thus, there exists a need for a process that operates at less than complete conversion, but allows for efficient use of the 2-keto-L-gulonic acid starting material and produces purified L-ascorbic acid in high yield. Accordingly, it is to the provision of such processes that the present invention is directed.
The present invention relates to a process for producing L-ascorbic acid which comprises the steps of subjecting an aqueous solution of 2-keto-L-gulonic acid (KLG) or derivatives of 2-keto-L-gulonic acid to an acid-catalyzed, or self-catalyzed cyclization, followed by separation of the product L-ascorbic acid and any unreacted 2-keto-L-gulonic acid compound so that the unreacted 2-keto-L-gulonic acid starting material may be effectively recycled. The process of the present invention provides methodologies for producing L-ascorbic acid in high yield by optimizing the formation of L-ascorbic acid while operating at a less than complete level of conversion of 2-keto-L-gulonic acid compounds. In this way, the decomposition of L-ascorbic acid product is minimized. The separation step is designed to provide an efficient and non-destructive isolation of unreacted 2-keto-L-gulonic acid starting material so that the 2-keto-L-gulonic acid can be further used for production of more L-ascorbic acid. The L-ascorbic acid isolated during the separation step can then be processed by crystallization or other methods to isolate L-ascorbic acid in its solid form.
In one aspect, the invention comprises a continuous process for manufacturing L-ascorbic acid comprising the steps of:
(a) heating in a reactor an aqueous solution of 2-keto-L-gulonic acid or derivatives of 2-keto-L-gulonic acid to form L-ascorbic acid at a conversion of less than 100 percent;
(b) continuously removing from the reactor a post-reaction solution comprising unreacted 2-keto-L-gulonic acid compound and L-ascorbic acid;
(c) continuously separating L-ascorbic acid from unreacted 2-keto-L-gulonic acid compound in the post-reaction solution to form an L-ascorbic acid rich solution and a solution rich in unreacted 2-keto-L-gulonic acid compound; and
(d) continuously recycling the solution rich in 2-keto-L-gulonic acid compound of step (c) back to the reactor.
In another aspect, the present invention comprises an ascorbic acid product made by the methods of the invention.
The present invention also comprises an apparatus for performing the methods of the invention. Thus, in another aspect, the present invention comprises a system for manufacturing L-ascorbic acid comprising:
(a) a reactor for conversion of 2-keto-L-gulonic acid to L-ascorbic acid;
(b) a conduit for the continuous removal of a post-reaction solution comprising unreacted 2-keto-L-gulonic acid and L-ascorbic acid from the reactor prior to complete conversion;
(c) a separation system for continuously separating L-ascorbic acid product from unreacted 2-keto-L-gulonic acid compound in the post-reaction solution to form an L-ascorbic acid rich solution and a 2-keto-L-gulonic acid rich solution;
(d) a conduit for transferring the 2-keto-L-gulonic acid rich solution back to the reactor;
(e) a conduit for transferring fresh 2-keto-L-gulonic acid to the reactor;
(f) a conduit for removing the L-ascorbic acid rich solution for subsequent purification and/or storage;
(g) at least one pump to pump reactants and products through the system; and
(h) at least one valve for controlling pressure throughout the system.
The foregoing focuses on the more important features of the invention in order that the detailed description which follows may be better understood and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention which will be described hereinafter and which will form the subject matter of the claims appended hereto. It is to be understood that the invention is not limited in its application to the specific details as set forth in the following description and figures. The invention is capable of other embodiments and of being practiced or carried out in various ways.
From the foregoing summary, it is apparent that an object of the present invention is provide efficient methods for the production of L-ascorbic acid. These, together with other objects of the present invention, along with various features of novelty which characterize the invention, are pointed out with particularity in the claims.